Many large consumers of oxygen, such as integrated steel mills, require both low purity oxygen and high purity oxygen. Because of the large volumes of oxygen required, entire cryogenic air separation plants are dedicated to providing the oxygen for such consumer. Typically two cryogenic air separation plants are employed, one for producing the high purity oxygen and the other for producing the low purity oxygen.
Both the high purity oxygen plant and the low purity oxygen plant must have a back up system in order to ensure that the flow of oxygen will continue to the steel mill in the event the oxygen plant must shut down. The back up system for the low purity oxygen plant is the high purity oxygen plant since a use that requires low purity oxygen can also operate using high purity oxygen without any loss of quality. However the high purity oxygen plant cannot be backed up by the low purity oxygen plant because a use that requires high purity oxygen cannot operate effectively with low purity oxygen. Accordingly, the back up system for the high purity oxygen plant is a tank filled with high purity liquid oxygen, which is vaporized and used if the need arises. This back up system, while necessary, is expensive to operate. It would be very desirable to have a low purity oxygen plant which, if the need arises, can quickly switch to producing, and can efficiently produce, high purity oxygen, so that such plant can serve as the back up to the high purity oxygen plant and thus eliminate the need for the expensive liquid oxygen reserve tank.
It is therefore an object of this invention to provide a cryogenic air separation plant which can produce efficiently either high purity oxygen or low purity oxygen and can quickly switch from producing one to the other.